1. Field of the Invention
This invention relates to a dropper type regulator that generates a desired voltage from a higher voltage.
2. Description of the Related Art
A common semiconductor integrated circuit and a conventional regulator circuit will be explained referring to FIGS. 3 and 4.
FIG. 3 is a layout showing the common semiconductor integrated circuit. An internal circuit 101 is disposed in a middle of an LSI chip 100 such as a microcomputer. The internal circuit 101 is composed of analog circuits and digital circuits. The internal circuit 101 is surrounded with circuits (hereafter collectively referred to as I/O circuits 102) serving as input circuits that receive input signals from outside of the LSI chip 100 and transfer them to the internal circuit 101 or serving as output circuits that output signals from the internal circuit 101 to external circuits. A predetermined power supply voltage Vdd that is necessary for operation of each of the circuits is supplied externally.
Some kinds of LSI chip 100 require generating a desired low voltage (3 volts, for example) suitable for driving the internal circuit 101 from the power supply voltage (5 volts, for example) used to drive the I/O circuits 102, in order to reduce power consumption. The dropper type regulator circuit is used to generate such a low voltage.
FIG. 4 is a circuit diagram showing a conventional dropper type regulator circuit. The regulator circuit is provided with a control MOS transistor 103 of P-channel type having a source to which the power supply voltage Vdd is applied, first and second resistors 104 and 105 connected in series with the control MOS transistor 103, an operational amplifier 106 having a first differential input terminal (−) to which a reference voltage Vref is applied, a second differential input terminal (+) to which a voltage Va at a connecting node between the first resistor 104 and the second resistor 105 is applied and a differential output terminal which is connected with a gate of the control MOS transistor 103. An output voltage Vout is obtained from a connecting node between the control MOS transistor 103 and the first resistor 104. The reference voltage Vref is generated by a bandgap reference voltage generation circuit 107 known in the art, for example. Technologies of the regulator circuit are disclosed in Japanese Patent Application Publication No. 2000-284843.
Not all circuits in a microcomputer are required to operate at any instance. Other than a normal operation state, there is a low power consumption state, such as a stand-by state, which includes various modes consuming various operating currents. For example, a CPU (Central Processing Unit) stops executing instructions in a HALT mode. In addition, supply of clocks to the other circuits is stopped in an IDLE mode. Furthermore, an oscillation of the system clock itself is stopped in a STOP mode.
In the conventional regulator circuit described above, however, an output transistor constituting the operational amplifier 106 and the control MOS transistor 103 are designed considering the maximum load current so that the predetermined voltage is stably maintained in the normal operation state. As a result, there arises a problem that an unnecessary operating current flows in the low power consumption state.